File transfers slow using WAN IP

I have a file server at 10.10.10.110:7500 inside my local network with a port forward for remote access. When I access the file server at the LAN IP, my transfer speed averages 12 MB/s. However, when I access it at the WAN IP from inside the LAN, my transfer speed averages 4 MB/s and the router's CPU load explodes (gets to like 6.00 or higher). I have NAT Loopback: All and NAT Target: Masquerade set in Advanced>Firewall.

My recommendation would be to cease this behaviour, as it equates to going to a doctor and stating "it hurts when I stab myself in the face with a fork":

When I access the file server at the LAN IP, my transfer speed averages 12 MB/s. However, when I access it at the WAN IP from inside the LAN, my transfer speed averages 4 MB/s and the router's CPU load explodes ...

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The problem has to do with the extra processing needed to deal with this kind of situation. There i a lot more than goes on than just re-writing the source/destination IPs in the IP header. I keep telling people on these forums to stop doing this.